It has been shown in U.S. Pat. No. 4,697,131 how to improve the operation of a voltage-source inverter and of a variable-frequency constant-voltage AC motor drive, by providing in the regenerating mode a current path for the negative DC-link current so as to criss-cross, or invert, current flow from the inverter side into the AC-to-DC converter terminals, and to sequentially interrupt such current path, concurrently with the triggering of a power switch in the converter firing sequence, thereby to create a dead-time in the commutation sequence. This is effected with the help of diodes, used in the forward mode to pass current directly from converter to inverter, and with the help of diagonally connected network including a GTO and an auxiliary thyristor, turned ON concurrently with the pair of thyristors next in the converter firing sequence, so as to cause reverse polarity in the DC-link, such GTO triggering being followed by current bypassing, with the assist of the diodes, when turning the GTO OFF, so as to create a dead-time before the subsequent thyristor firing in the converter sequence. For the purpose of this description, U.S. Pat. No. 4,697,131 is hereby incorporated by reference.
The operation of such a motor drive can still be improved. One object is to provide protection against current surges which might cause fuse blowing. Another object is to increase the capability of the circuit by boosting the DC-link voltage so as to maximize inverter and motor usage under a given AC power supply voltage.
It has also been shown in U.S. Pat. No. 4,788,635 that a GTO device has been added to a voltagesource inverter and controlled with a duty cycle in the regenerating mode, in order to match the voltage on the inverter side of the DC-link with the voltage on the mains converter side, thereby solving the disparity between the voltage available from the AC-to-DC converter in the motoring mode and the voltage from the converter under natural commutation in the regenerating mode.